Oranges generally began to fall off the trees when they are ripe.
You can't tell by color, you can test the fruit to check on it's ripeness.
Lemons are among the most difficult to determine ripeness.
It can take 6 to 9 months for Lemons to ripen, depending on the conditions.
Warmth and plenty of water encourage the fruit to ripen as quickly as possible.
Lemons stay firm until they are overripe. Look for a slightly softened texture and fruit that is heavier then it appears. This indicate a fruit heavy with juice.
Because Lemons can produce flowers and fruit as long as the weather conditions are right, your tree may have fruit at various stages of ripeness.
The fruit you are referring too, may not be as heavy as past years.